1. Field of the Invention
The present invention relates to a semiconductor device, and in particular, to a semiconductor device that produces a depletion layer in a channel of a transistor.
2. Description of the Background Art
Generally, as for a passive element such as a diode and a resistance, the more voltage is applied, the more current flows through the element. As for a diode, in particular, the more voltage is applied, the larger an increase in current becomes. In a bootstrap diode, for example, a relatively large voltage is applied instantaneously during power-on until a capacitor is charged, and thus, a large current flows through the diode. On the other hand, in the normal operation state, a voltage of only approximately several volts is applied, and thus, a small current flows through the diode. In the bootstrap diode, a resistance is serially connected to the diode in order to cancel the characteristic of the diode that the more voltage is applied, the larger an increase in current becomes.
A component for limiting a current flow includes, for example, a current-limiting semiconductor switch. In Japanese National Patent Publication No. 2000-505239, for example, the current-limiting semiconductor switch is proposed.
The characteristics desired in the bootstrap diode are to avoid the large current during power-on and to decrease the loss during the normal operation or increase the operation speed during the normal operation. In order to have such characteristics, desired is the characteristic of reasonably large current at low voltage and small current at high voltage. In the conventional bootstrap diode, however, this characteristic cannot be obtained because the resistance is serially connected to the diode.